This was the first primary endorsement made by Cruz, who has a strong following among tea party-aligned Republicans. The endorsement allows him to honor his pledge not to back insurgents in primaries involving Senate GOP colleagues, The Washington Post reported.

Shannon is in an open primary. A special election in November will fill the seat being vacated by Republican Sen. Tom Coburn two years before his term ends in 2016.

Cruz plans to attend an April 24 campaign rally for Shannon in Tulsa. Shannon also has the backing of 2008 GOP vice presidential candidate Sarah Palin and Utah Republican Sen. Mike Lee. All three are expected to attend the Tulsa event, according to The Hill.

The tea party-aligned Senate Conservatives Fund is also backing Shannon.

In a statement released by Shannon's office, Cruz said Shannon "is a strong constitutional conservative."

Rep. James Lankford, a two-term GOP congressman whose district includes Oklahoma City, is regarded as the candidate favored by the establishment wing of the party. He also has the endorsement of former Arkansas governor Mike Huckabee, Roll Call reported.

A March poll conducted by a firm aligned with Shannon shows Lankford with a shrinking lead at 37 percent, Shannon with 28 percent, and Randy Brogdon, a former state senator Randy Brogdon garnering 7 percent, the Post reported.

Shannon is an African American whose heritage also includes Chickasaw ancestry, according to The Hill.

The primary is set for June 24. The winner is expected to be victorious on Nov. 4 because Oklahoma leans heavily Republican.